The present invention pertains generally to rf antennas. More particularly, the present invention pertains to rf antennas that are useful for generating plasmas. The present invention is particularly, but not exclusively, useful as a shielded rf antenna which is protected from the adverse effects of plasma deposits during its operation.
Radio frequency (rf) power is a well known means for producing and heating the plasmas that are needed for processing materials or separating masses. There are, however, limitations to the use of rf power for these purposes. In some applications, such as where there are high levels of radiation, or the size of the plasma chamber is particularly large, the power requirements for generating and maintaining a suitable plasma are very high. Moreover, it happens that for high power applications, conventional rf antenna configurations are susceptible to breakdown between the antenna elements and the adjacent grounded structure. Another problem is encountered when deposits from the plasma develop on the rf antenna and absorb or prevent the transmission of rf power from the antenna.
One solution to the problems mentioned above is to house the rf antenna elements in a separate compartment that will isolate the elements from the plasma. Such a compartment can then be pumped or pressurized, as required, to avoid the breakdown between the antenna elements and the grounded structure. For this purpose, it is known that certain ceramics have sufficient structural strength to withstand the pressure differentials that may be used. Further, many of these ceramics are known to be generally transparent to rf power. It is a concern, however, that the ceramic facing of such a compartment is particularly susceptible to the accumulation of depositions from a plasma when it is in direct contact with the plasma. The consequence then is that the depositions on the ceramic facing absorb rf power. As indicated above, this will eventually cause the rf antenna to become ineffective. This problem is particularly acute when the plasma is a metallic plasma.
In light of the above, it is an object of the present invention to provide a device for generating a plasma which uses a ceramic shield to encase or enclose an antenna element that is capable of generating very high rf powers. It is another object of the present invention to provide a device for generating a plasma which incorporates a Faraday shield (cage) to effectively protect the ceramic shield from material deposits that can absorb rf power and thereby significantly diminish the effectiveness of the device. Yet another object of the present invention is to provide a device for generating a plasma that is relatively easy to manufacture, is simple to use and is comparatively cost effective.
In accordance with the present invention, a device for generating a plasma includes an enclosed chamber, and an antenna that is positioned adjacent to the inside wall of the chamber. As intended for the present invention, the chamber is substantially cylindrical and defines a longitudinal central axis with the antenna centered on the axis. Importantly, elements of the antenna are positioned around the chamber inside a substantially annular shaped pressure compartment.
For its construction, the pressure compartment housing the antenna elements is formed at the wall of the chamber and includes a pair of metal side walls. More specifically, the metal side walls are substantially parallel to each other, and they extend inwardly from the wall of the chamber toward the central axis. A ceramic shield is attached to the extended edges of the side walls and, with the ceramic shield so attached, the pressure compartment is formed. Structurally, the pressure compartment is established between opposed side walls, and between the ceramic shield and the wall of the chamber.
As mentioned above, the antenna elements of the present invention are located inside the pressure compartment, between the shield and the wall of the chamber. This is done so that the ceramic shield will effectively isolate the antenna elements from the inside of the plasma chamber. Specifically, as will be appreciated by the skilled artisan, an isolated antenna element inside the pressure compartment can operate in a different, more favorable pressure environment for the antenna than that which may be required in the plasma chamber itself. Further, the ceramic shield allows the antenna elements to radiate the rf power into the chamber. Preferably, the ceramic shield is made of a material that will be effectively transparent to rf energy, such as quartz. Behind the ceramic shield, the antenna will preferably include a plurality of elements that are substantially circular loops that are connected to a source of alternating voltage. Power from this voltage source can then be radiated from the rf antenna element into the chamber. For specific applications, the voltage source may also be used to control the relative phase of the rf power in adjacent loops of the antenna.
In addition to the antenna elements that are located inside the pressure compartment, the device of the present invention also includes a barrier that is mounted on the device outside the pressure compartment, but inside the chamber. This barrier is preferably a structure of a type commonly known as a non-transparent Faraday shield (cage). As used for the present invention, this barrier is positioned with the antenna element and ceramic shield between the barrier and the wall of the chamber. Thus, in a radial direction toward the axis of the chamber, the device of the present invention includes the wall of the chamber, an antenna element, a ceramic shield, and a barrier. The plasma chamber is then established between the barrier and the axis of the chamber.
In its preferred embodiment, the barrier of the present invention includes an inner perforated layer having a plurality of elongated openings and an outer perforated layer also having a plurality of elongated openings. Both of these layers, the inner and the outer layers, are substantially cylindrical in shape and they are coaxially oriented along the central axis of the chamber with the inner layer being closest to the axis. The elongated openings through both the inner layer and the outer layer are aligned generally parallel to the central axis of the chamber. Furthermore, the openings through the inner layer are offset from the openings through the outer layer such that, in a radial direction from the central axis, the ceramic shield is blocked from the chamber by at least one of the layers.
During an operation of the device of the present invention, the antenna elements are energized by an alternating voltage source. The resultant rf power from the antenna is then radiated through the ceramic shield, past the barrier (Faraday shield) and into the plasma chamber. Inside the plasma chamber, the rf power will then interact with vapors to create a plasma. It will happen that as the plasma is created inside the chamber by the rf power from the antenna, the plasma will include material components that can precipitate out of the plasma. The barrier (Faraday shield), however, prevents material components in the plasma from being deposited on the ceramic shield. Thus, the rf power is not affected by material deposits on the ceramic shield which could, otherwise, absorb the rf power and significantly diminish the effectiveness of the antenna element.